The technique of cutting and/or machining using a jet of liquid at a very high pressure consists in spraying liquid at a pressure of from 1000 to 8000 bar. The liquid, generally pure water or water with additives is then ejected at a very high speed, from 600 to more than 1000 m/s, and directed onto the piece to be worked. Thus, the technique of machining using a jet of liquid at a very high pressure makes it possible to work numerous materials such as plastics, paper or metal alloys, and to do so without emitting dust or generating heat. In order to make it easier to cut the materials, abrasive particles may be added to the sprayed water.
The cutting heads used according to this cutting method conventionally comprise a collimation tube, a nozzle with a small internal diameter, a mixing chamber and a gun for spraying liquid at a very high pressure.
The liquid at a very high pressure enters through the collimation tube. The liquid is then sprayed through the nozzle and enters at a high speed into the mixing chamber provided with an abrasive particle inlet. The mixture of liquid and the abrasive particles is concentrated and the jet at a very high pressure is directed onto the piece to be worked by a gun with a small internal diameter. This gun is generally known by the term focusing gun.
Conventionally, such focusing guns are manufactured by machining from a solid piece.
These focusing guns, however, are pieces which have a large ratio of length to diameter, which makes it difficult to machine a duct from a solid piece. In particular, the machining does not make it possible to go below a certain internal diameter threshold for a given length. Yet the internal diameter of the focusing gun determines the precision of the liquid jet.
Furthermore, according to this manufacturing technique, the focusing guns are normally made of tungsten carbide. This is because this ceramic is easy to use and has a reasonable cost for such an application. However, such guns made of tungsten carbide can be used only with abrasives of the oxide type, such as garnet or aluminum oxide with a low relative hardness. This is because the use of abrasives with a hardness greater than that of tungsten carbide, from 8 to 9 on the Mohs scale, would rapidly wear the interior of the gun, leading to a premature loss of precision and a lifetime incompatible with the usual applications of machines for machining using a jet of liquid at a very high pressure.
Another method for manufacturing focusing guns consists in vapor depositing a ceramic on a cylindrical graphite support, then removing the cylindrical support by heating once the ceramic has formed a tubular structure around the graphite.
This method makes it possible in particular to manufacture guns made of silicon carbide, a ceramic which has a high hardness of the order of 9.5 on the Mohs scale, making it possible to work with a wide range of abrasives which are no longer limited to oxides.
This manufacturing method, however, turns out to be complex to implement and the production cost of the guns thus produced is high.
In order to overcome the problems associated with the length to internal diameter ratio of the gun, it has been envisaged to manufacture a focusing gun by assembling a plurality of elements. These elements may be cylindrical pieces of small height, and therefore with a low length to internal diameter ratio, which are aligned along the axis of the duct of the gun and kept assembled by clamping with an external piece of the sleeve type. This technique, however, is virtually unused. This is because it does not allow the various elements of the gun to be aligned correctly, and the alignment differences lead to perturbations of the flow of liquid at a very high pressure. Furthermore, the junction regions prove particularly sensitive to wear.
With a view to overcoming the drawbacks of the techniques above, another manufacturing method has consisted, as described particularly in U.S. Pat. No. 5,785,582 or DE 196 40 920, in producing focusing guns from two pieces provided with assembly faces by which they are intended to be joined in order, in the assembled state, to form a focusing gun. According to this method, a central groove is formed in the assembly face of each of the two pieces, this being formed in a planar median region of said assembly face and adapted to extend between the two ends thereof, said central grooves being adapted to form the duct of the focusing gun in the assembled position of the two pieces.
The problem which such a technique needs to resolve resides in obtaining precise positioning of the two pieces, making it possible to obtain a duct having perfectly controlled internal dimensions. At present, however, no solution makes it possible to resolve this problem satisfactorily.
Specifically, the current methods either require an expensive external mechanism (techniques described in DE 297 02 397 or U.S. Pat. No. 2,332,407), or do not guarantee perfect relative positioning and perfect holding of the two pieces during the phase of assembling them:
technique using a film enclosing the two pieces described in U.S. Pat. No. 5,785,582
technique of bonding the assembly faces of the two pieces, described in DE 196 40 920, according to which:
in the assembly face of at least one of the two pieces, laterally with respect to the planar median region thereof, at least one bonding reservoir is formed, which is intended to be filled with a quantity of adhesive substance adapted to adhere to the portion of the assembly face of the other piece lying opposite said reservoir, in the assembled position of the two pieces,
each bonding reservoir is filled with adhesive substance, and the two pieces are placed in contact so that they are joined by their assembly face and connected by means of the adhesive substance.